One type of image difference detector includes means for holding two created image representations, such as transparencies formed by photographs taken on slide film, so they are coplanar. Collimated light passes through the two transparencies and the light is focused as images at an image plane. The images are made coincident at the image plane by polarizing the light passing through the two images in transverse directions and passing the light through a Wollaston prism that forms coincident images. The Wollaston prism employs the polarizations to direct the images into coincidence. The polarization also results in a 180.degree. phase difference between the images. An analyzer can sense locations where the two coincident images do not cancel, to thereby sense areas of the images which are not coincident. A modified form of the image difference detector employs different color filters in line with the transparencies, such as green and red filters, to create coincident images at the image plane where the image is yellow except at areas where the images are not identical (where they are red or green).
Devices for creating image representations such as transparencies representing the images being compared, which could generate the image representations in real time and at low cost, would facilitate the production of real-time optical processing devices.